Several revisions in classifying the great apes have caused the use of the term "hominid" to vary over time. Its original meaning referred only to humans (Homo) and their closest extinct relatives. That restrictive meaning has now been largely assumed by the term "hominin", which comprises all members of the human clade after the split from the chimpanzees (Pan). The current, 21st-century meaning of "hominid" includes all the great apes including humans. Usage still varies, however, and some scientists and laypersons still use "hominid" in the original restrictive sense; the scholarly literature generally shows the traditional usage until around the turn of the 21st century.[4]

Within the taxon Hominidae, a number of extant and known extinct, that is, fossil, genera are grouped with the humans, chimpanzees, and gorillas in the subfamily Homininae; others with orangutans in the subfamily Ponginae (see classification graphic below). The most recent common ancestor of all Hominidae lived roughly 14 million years ago,[5] when the ancestors of the orangutans speciated from the ancestral line of the other three genera.[6] Those ancestors of the family Hominidae had already speciated from the family Hylobatidae (the gibbons), perhaps 15 million to 20 million years ago.[6][7]

At sites far distant from East Africa, the presence of other generalized non-cercopithecids, that is, non-monkey primates, of middle Miocene age—Otavipithecus from cave deposits in Namibia, and Pierolapithecus and Dryopithecus from France, Spain and Austria—is further evidence of a wide diversity of ancestral ape forms across Africa and the Mediterranean basin during the relatively warm and equable climatic regimes of the early and middle Miocene. The most recent of these far-flung Miocene apes (hominoids) is Oreopithecus, from the fossil-rich coal beds in northern Italy and dated to 9 million years ago.

Molecular evidence indicates that the lineage of gibbons (family Hylobatidae), the lesser apes, diverged from that of the great apes some 18–12 million years ago, and that of orangutans (subfamily Ponginae) diverged from the other great apes at about 12 million years. There are no fossils that clearly document the ancestry of gibbons, which may have originated in a still-unknown South East Asian hominoid population; but fossil proto-orangutans, dated to around 10 million years ago, may be represented by Sivapithecus from India and Griphopithecus from Turkey.[8]

Species close to the last common ancestor of gorillas, chimpanzees and humans may be represented by Nakalipithecus fossils found in Kenya and Ouranopithecus found in Greece. Molecular evidence suggests that between 8 and 4 million years ago, first the gorillas (genus Gorilla), and then the chimpanzees (genus Pan) split off from the line leading to the humans. Human DNA is approximately 98.4% identical to that of chimpanzees when comparing single nucleotide polymorphisms (see human evolutionary genetics).[9] The fossil record, however, of gorillas and chimpanzees is limited; both poor preservation—rain forest soils tend to be acidic and dissolve bone—and sampling bias probably contribute most to this problem.

Other hominins probably adapted to the drier environments outside the African equatorial belt; and there they encountered antelope, hyenas, elephants and other forms becoming adapted to surviving in the East African savannas, particularly the regions of the Sahel and the Serengeti. The wet equatorial belt contracted after about 8 million years ago, and there is very little fossil evidence for the divergence of the hominin lineage from that of gorillas and chimpanzees—which split was thought to have occurred around that time. The earliest fossils argued by some to belong to the human lineage are Sahelanthropus tchadensis (7 Ma) and Orrorin tugenensis (6 Ma), followed by Ardipithecus (5.5–4.4 Ma), with species Ar. kadabba and Ar. ramidus.

Taxonomic history

Terminology

The classification of the great apes has been revised several times in the last few decades; these revisions have led to a varied use of the word "hominid" over time. The original meaning of the term referred to only humans and their closest relatives—what is now the modern meaning of the term "hominin". The meaning of the taxon Hominidae changed gradually, leading to a different (modern) usage of "hominid" that today includes all the great apes including humans.

The term hominid is easily confused with a number of very similar words:

A hominoid, commonly called an ape, is a member of the superfamily Hominoidea: extant members are the gibbons (lesser apes, family Hylobatidae) and the hominids.

A hominid is a member of the family Hominidae, the great apes: orangutans, gorillas, chimpanzees, and humans.

A hominine is a member of the subfamily Homininae: gorillas, chimpanzees, and humans (excludes orangutans).

A homininan, following a suggestion by Wood and Richmond (2000), would be a member of the subtribe Hominina of the tribe Hominini: that is, modern humans and their closest relatives, including Australopithecina, but excluding chimpanzees.[10]

Extant and fossil relatives of humans

As mentioned, Hominidae was originally the name given to the family of humans and their (extinct) close relatives, with the other great apes (that is, the orangutans, gorillas, and chimpanzees) all being placed in a separate family, the Pongidae. However, that definition eventually made Pongidae paraphyletic because at least one great ape species (the chimpanzees) proved to be more closely related to humans than to other great apes. Most taxonomists today encourage monophyletic groups—this would require, in this case, the use of Pongidae to be restricted to just one closely related grouping. Thus, many biologists now assign Pongo (as the subfamily Ponginae) to the family Hominidae. The taxonomy shown here follows the monophyletic groupings according to the modern understanding of human and great ape relationships.

Humans and close relatives including the tribes Hominini and Gorillini form the subfamily Homininae (see classification graphic below). (A few researchers go so far as to refer the chimpanzees and the gorillas to the genus Homo along with humans.) [11][12][13] But, it is those fossil relatives more closely related to humans than the chimpanzees that represent the especially close members of the human family, and without necessarily assigning subfamily or tribal categories.[14]

The exact criteria for membership in the tribe Hominini under the current understanding of human origins are not clear, but the taxon generally includes those species that share more than 97% of their DNA with the modern human genome, and exhibit a capacity for language or for simple cultures beyond their 'local family' or band. The theory of mind concept—including such faculties as empathy, attribution of mental state, and even empathetic deception—is a controversial criterion; it distinguishes the adult human alone among the hominids. Humans acquire this capacity after about four years of age, whereas it has not been proven (nor has it been disproven) that gorillas or chimpanzees ever develop a theory of mind.[16] This is also the case for some New World monkeys outside the family of great apes, as, for example, the capuchin monkeys.

However, even without the ability to test whether early members of the Hominini (such as Homo erectus, Homo neanderthalensis, or even the australopithecines) had a theory of mind, it is difficult to ignore similarities seen in their living cousins. Orangutans have shown the development of culture comparable to that of chimpanzees,[17] and some[who?] say the orangutan may also satisfy those criteria for the theory of mind concept. These scientific debates take on political significance for advocates of great ape personhood.

Phylogeny

Below is a cladogram with extinct species.[18] It is indicated approximately how many million years ago (Mya) the clades diverged into newer clades.[19]

Taxonomy of Hominoidea (emphasis on family Hominidae): after an initial separation from the main line by the Hylobatidae (gibbons) some 18 million years ago, the line of Ponginae broke away, leading to the orangutan; later, the Homininae split into the tribes Hominini (led to humans and chimpanzees) and Gorillini (led to gorillas).

Model of the phylogeny of Hominidae, with adjacent branches of Hominoidea, over the past 20 million years.

Extant

There are eight living species of great ape which are classified in four genera. The following classification is commonly accepted:[1]

Family Hominidae: humans and other great apes; extinct genera and species excluded[1]

Red Deer Cave people† (scientific name has not yet been assigned; perhaps a race of modern humans or a hybrid[27] of modern humans and Denisovans[28])

Physical description

The great apes are large, tailless primates, with the smallest living species being the bonobo at 30–40 kilograms in weight, and the largest being the eastern gorillas, with males weighing 140–180 kilograms. In all great apes, the males are, on average, larger and stronger than the females, although the degree of sexual dimorphism varies greatly among species. Although most living species are predominantly quadrupedal, they are all able to use their hands for gathering food or nesting materials, and, in some cases, for tool use.[29]

Most species are omnivorous,[citation needed] but fruit is the preferred food among all but some human groups. Chimpanzees and orangutans primarily eat fruit. When gorillas run short of fruit at certain times of the year or in certain regions, they resort to eating shoots and leaves, often of bamboo, a type of grass. Gorillas have extreme adaptations for chewing and digesting such low-quality forage, but they still prefer fruit when it is available, often going miles out of their way to find especially preferred fruits. Humans, since the neolithic revolution, consume mostly cereals and other starchy foods, including increasingly highly processed foods, as well as many other domesticated plants (including fruits) and meat. Hominid teeth are similar to those of the Old World monkeys and gibbons, although they are especially large in gorillas. The dental formula is 2.1.2.32.1.2.3. Human teeth and jaws are markedly smaller for their size than those of other apes, which may be an adaptation to eating cooked food since the end of the Pleistocene.[30][31]

Gestation in great apes lasts 8–9 months, and results in the birth of a single offspring, or, rarely, twins. The young are born helpless, and require care for long periods of time. Compared with most other mammals, great apes have a remarkably long adolescence, not being weaned for several years, and not becoming fully mature for eight to thirteen years in most species (longer in humans). As a result, females typically give birth only once every few years. There is no distinct breeding season.[29]

The gorillas and the common chimpanzee live in family groups of around five to ten individuals, although much larger groups are sometimes noted. Chimpanzees live in larger groups that break up into smaller groups when fruit becomes less available. When small groups of female chimpanzees go off in separate directions to forage for fruit, the dominant males can no longer control them and the females often mate with other subordinate males. In contrast, groups of gorillas stay together regardless of the availability of fruit. When fruit is hard to find, they resort to eating leaves and shoots. Because gorilla groups stay together, the male is able to monopolize the females in his group. This fact is related to gorillas' greater sexual dimorphism relative to that of chimpanzees; that is, the difference in size between male and female gorillas is much larger than that of male and female chimpanzees. This enables gorilla males to physically dominate female gorillas more easily. In both chimpanzees and gorillas, the groups include at least one dominant male, and females leave the group at maturity.

Notes

^"Great ape" is a common name rather than a taxonomic label, and there are differences in usage, even by the same author. The term may or may not include humans, as when Dawkins writes "Long before people thought in terms of evolution ... great apes were often confused with humans"[2] and "gibbons are faithfully monogamous, unlike the great apes which are our closer relatives."[3]